join method

String join ([String separator = "" ])

Converts each element to a String and concatenates the strings.

Iterates through elements of this iterable, converts each one to a String by calling Object.toString, and then concatenates the strings, with the separator string interleaved between the elements.

Implementation 

String join([String separator = ""]) {
  if (length == 0) return "";
  StringBuffer buffer = new StringBuffer()..writeAll(this, separator);
  return buffer.toString();
}